In the field of optical telecommunications, information is transmitted optically over a network of single-or multi-mode fibers. Many of the switching and splitting functions in the network are accomplished in free space. For free space propagation, the light is allowed to exit the fiber, typically a single mode fiber, and is then collimated by a lens. The collimated light beam then interacts with the switching and/or splitting elements, such as a narrowband interference filter. Once the free space interaction is complete, the light is then focused back down to a fiber for continued guided propagation.
One type of free-space splitting element commonly used in optical communications applications is a thin film, narrowband filter, which typically is highly reflecting for a particular channel in a multiple channel optical signal. The filter is used to reflect light in the wavelength band of the particular channel, while transmitting the remainder of the light. The free-space reflected light and transmitted light are typically then focused into respective fibers once the particular channel has been split out of the multiple channel signal.
The definitions of channel wavelength and bandwidth set by the International Telecommunications Union (ITU) are relatively strict, resulting in strict tolerances on the bandwidth and center wavelength of the thin film filter. Thin film interference filters that exactly match the center wavelength and bandwidth at a particular angle of incident light, however, are expensive to manufacture. One approach to reducing the expense is to use a less expensive filter, for which the center wavelength is less precise, and to vary the angle at which the light is incident on the filter. This process is referred to as angle tuning, and is often recommended by manufacturers of thin film filters.
In some geometries, the filter is angle tuned by rotating the filter about an axis perpendicular to the direction of propagation of the light. In another geometry, the incoming light is delivered to the filter from a fiber that is held by a ferrule in an off-axis position relative to the collimating lens. The angle of incidence can be adjusted by changing the offset of the fiber from the lens axis. This requires that the fiber be held in an adjustable ferrule, or that a number of ferrules with different offsets are available for putting the device together.